1. Field of the Invention
The present invention relates to a multi-beam optical head for recording information on a recording medium and/or for reproducing information from the recording medium, by using a plurality of light beams.
2. Description of the Related Art
Conventionally, an optical information recording and reproducing apparatus, such as a magneto-optic disk apparatus, has been utilized for an optical file and an external memory device of a computer, because it has a large storage capacity.
In recent years, using the optical information recording and reproducing apparatus as a memory device in a motion picture system, such as a high-definition TV, has been largely required. In the motion picture system, the memory device needs to have a higher data transmission rate. In order to transmit data at a higher rate, a method for recording the information on a plurality of tracks at one time or for reproducing the information recorded on the tracks in parallel is proposed. In this method, a plurality of light beams are irradiated onto a predetermined number of tracks of the recording medium, and then the information is recorded on and/or reproduced from the irradiated tracks. Hereinafter, the optical information recording and reproducing apparatus using a plurality of light beams is referred to as "a multi-beam optical head."
FIG. 6 schematically shows an exemplary configuration of a conventional multi-beam optical head which is used for a magneto-optic disk. This conventional multi-beam optical head includes three laser diodes as light-emitting devices. The three laser diodes are arranged in a line within a light source 1. As shown in FIG. 6, three light beams emitted from the three laser diodes are collimated by a collimator lens 2, and then enter a shaping prism 3. The shaping prism 3 transforms the distribution of light intensity of each of the three collimated light beams into an approximately round-shaped one. The shaped beams pass through a beam splitter 4 and an image rotatory prism 6 in this order. Then, after being reflected by a reflecting mirror 7 toward a recording medium 5, the shaped beams are converged onto the recording medium 5 by an objective lens 8. In this way, three beams are irradiated onto the recording medium 5 at one time to form three light spots.
A beam spot line constituted by the three light beams irradiated onto the recording medium 5 can be rotated around a rotation axis by rotating the image rotatory prism 6 around this rotation axis by means of a controller (not shown). The rotation axis is parallel to the optical axis of the three light beams.
On the other hand, the three light beams reflected by the recording medium 5 are collimated by the objective lens 8 again; reflected by the reflecting mirror 7; and passed through the image rotatory prism 6 so as to be incident on the beam splitter 4. The reflected light beams are separated from the beams being directed toward the recording medium 5 by the beam splitter 4. Thereafter, each of the beams is split by a Wollaston prism 9 into three light beams having respectively different directions of polarization. Furthermore, the split light beams are directed through a spot lens 10 and a cylindrical lens 11 to a photodetector 12 so as to be optically detected by the photodetector 12.
Based on the detection signal output from the photodetector 12 for detecting these split light beams, a focusing error signal and a tracking error signal are generated in a signal processing circuit (not shown) by an astigmatic method and a push-pull method, respectively.
In accordance with the focus error signal, the position of the objective lens 8 in the direction perpendicular to the recording medium 5 is controlled so as to form focal points on the medium 5. On the other hand, the tracking error signal is used for controlling a positional relationship between the beam spots and the predetermined tracks. The deviation of the three beam spots from the predetermined tracks may occur due to the exchange of the recording medium 5 or the like. In this case, a beam spot line including the beam spots is rotated by rotating the prism 6 based on the tracking error signal so that all of the beam spots are precisely positioned on the predetermined tracks. FIG. 7 illustrates an example in which two spots deviate from the corresponding tracks. In FIG. 7, a beam spot line including three beam spots are at an angle of .DELTA..theta..sub.3 with the predetermined tracks. By rotating the prism 6, the beam spot line is rotated to be at an angle of .DELTA..theta..sub.2 with the predetermined tracks, as shown in FIG. 8, so that all of the three beam spots are precisely positioned on the predetermined tracks.
As described above, in the above-mentioned conventional apparatus using multiple beams, the image rotatory prism 6 is required for precisely positioning the beam spots on the predetermined tracks on the recording medium 5, whereas the image rotatory prism 6 is not necessary for an optical head using only one beam. Therefore, the weight of the optical head using multiple beams becomes disadvantageously heavy and the necessary cost increases.
The operation of the image rotatory prism 6 will be described. If the image rotatory prism 6 is rotated around the axis AB parallel to the base of the prism 6 as indicated by the arrow shown in FIG. 9A, then the light beams, e.g., three beams which converged to form three beam spots .alpha., .beta. and .gamma. on the recording medium 5, as shown in FIG. 9C, are rotated so that the beam spots .alpha., .beta. and .gamma. are displaced to the beam spots .alpha.', .beta.' (=.beta.) and .gamma.' around the center spot .beta. as shown in FIG. 9B. In this case, it is essential that the respective beams are parallel to the rotation axis AB and a central one of the light beams coincides with the rotation axis AB.
On the other hand, if the light beams are not parallel to the rotation axis AB, or the central beam does not coincide with the rotation axis AB, the center beam spot .beta. may be displaced to the different spot .beta.", as shown in FIG. 9D. As a result, all of the beam spots .alpha., .beta. and .gamma. are moved to the spots .alpha.", .beta." and .gamma." by rotation.
Therefore, in the multi-beam optical head including the image rotatory prism 6, the above-mentioned control of the positional relationship between the beam spot line consisting of the beam spots and the predetermined tracks can be performed only when the rotation axis AB of the prism 6 is in parallel to the prism 6 itself and the central beam coincides with the rotation axis AB. This makes the arrangement of optical devices in the multi-beam optical head complicated. Also, optical adjustment of the optical devices is made more difficult because the adjustment must be performed with precision. As a result, the weight of whole optical head increases.